U.S. patent application Ser. No. 07/714,816 filed Jun. 13, 1991 by R. H. Taylor et al., and herein incorporated by reference, disclosed a system for the augmentation of surgery with a remote center-of-motion manipulator structure with orthogonally decoupled degrees of freedom resolved at a work point frame distal to the manipulator structure. In such a mechanism, successive axes of revolute motion are perpendicular to each other and intersect at the work point, and the linear motion axes are similarly perpendicular. This orthogonal decoupling at a remote tool frame has many advantages for surgical applications, both in "passive" and "active" embodiments of the structure.
In passive embodiments, each manipulator degree-of-freedom may be equipped with a brake but all motive force is provided by a human surgeon. In typical applications, the surgeon wishes to manipulate a surgical tool, tool guide, or piece of the patient's anatomy (such as a bone fragment) so that a desired spatial relationship relative to the patient or equipment in the operating room is achieved. Since each motion axis of the mechanism only affects one rotational or translational degree of freedom of a bone fragment or other object rigidly held at the work point, this structure permits the surgeon to work on only one or two degrees of freedom at a time without disturbing those which have already been aligned, thus greatly simplifying his alignment task while allowing him (or her) to work in a coordinate system that is natural and intuitively understandable for the task at hand.
In active embodiments, a computer controlled actuator drives one or more of the motion axes. Typical surgical applications include manipulation of laparoscopic cameras, precise tissue removal, biopsy sampling, and the like. Many of these applications require accurately controlled motion under computer control, often involving reorientation of a tool, while strictly limiting undesired motions. Further, many of these applications only require relatively slow precise computer-controlled motions, with somewhat more rapid approximate gross motions, which may be done passively.
In the example of laparoscopic camera manipulation, for example, the principal requirements for the manipulation system include safety, the ability to limit lateral motion at the point where the camera enters the body, the ability to rapidly achieve an approximate gross position of the camera, and the ability to make precise, calibrated motions of the camera under control of the computer.
An orthogonally decoupled remote center-of-motion (RCM) structure has many advantages for filling these requirements, as compared to a traditional industrial robot for which the design point is typically the ability to move rapidly through a large work volume with a compact (usually serial-link) mechanical structure. In a conventional industrial robot, motions about a remote motion center are achieved by coordinated motions of multiple joints, many of which may be required to make fairly large motions in order to achieve relatively small tool reorientations. If the joints are fast enough to achieve these motions in reasonable time, they are also typically fast enough to cause quite rapid end effector motion when the manipulator is at a different position. Put another way, if the joint actuators (motors and transmission ratios) are sized so as to limit the maximum possible speed of the tool or other critical point in all circumstances (often an important safety consideration) then many desired motions may be excruciatingly slow. A decoupled RCM structure positioned so that the motion center coincides with the point where the camera enters the patient's body avoids many of these problems. Each actuator can be sized to produce the desired rate of motion in the corresponding degree of freedom. Furthermore, it is easy to permit more rapid manual positioning of the entire mechanism or of selected subsets of the degrees of freedom by means of manually actuated clutches. In this case, it is important to note that the kinematic constraints imposed by the mechanism prevent inadvertent lateral motions when changing the orientation of the camera.
One embodiment of this concept uses crossed goniometer axes to achieve the requisite decoupling of revolute motions at a remote center. While this embodiment has many advantages, it also has several drawbacks. The principal difficulty is a size/working radius trade off. If a large working radius is desired, then the goniometer axes must be quite large, and the resulting robot structure can become somewhat clumsy and obtrusive in the operating room or can impede access to the patient. If the working radius is small, then the mechanism may get in the way of the surgeon's hands, instruments, or direct vision. A related consideration is that high quality goniometer axes can be expensive and difficult to fabricate.